High frequency amplifier



Nov. 12, 1935. P. KAPTEYN 2,021,103

HIGH FREQUENCY AMPLIFIER Original Filed Feb. 2, 1932 Patented Nov. 12,1935 UNITED STATES PATENT OFFICE.

HIGH FREQUENCY AMPLIFIER Paul Kapteyn, Berlin, Germany ruary 16, 1931 5Claims.

In high frequency amplifiers it is important, for the purpose ofobtaining maximum amplification per stage (aperiodic high frequencyamplification) or the greatest possible independence 5, of frequency toemploy tubes having the smallest possible capacities between the singleelectrodes, which do not enter into the oscillation circuit capacity ofoscillatory circuits, but are switched in parallel to chokes orresistances. The accomplishment of small capacities is successful to avery considerable degree by the use of screening grid tubes, as in thecase of tubes of this nature the reaction of the anode causing in gridcircuits the occurrence of the apparent capacity, increased in relationto the static grid anode capacity to the extent of the amplificationfactor, is capable of being reduced to an ineffective fraction of itsnormal value.

In itself, however, the inclusion of auxiliary grids is accompanied bythe disadvantage that the total of the static capacities is increased.For this reason, generally speaking, the use of screening grid tubes hasnot heretofore been taken into consideration in the case of aperiodichigh frequency amplifiers.

According to the invention amplifier tubes may be used which fulfill toa very extensive degree the above requirements. The invention is basedon the recognition that the total of the detrimental capacities of eachanode circuit, and accordingly-so far as the tube itself is concernedthe total of the electrode capacities efiective between the anode andthe cathode, should be kept small.

If now two detrimental capacities are formed by three electrodes, which,for example, are coaxial, whereby the one capacity is naturally formedby the outer and the middle electrode, and the second by the middle andthe inner electrode, a certain dimensioning of the middle electrode willexist with given inner and outer electhe inner electrode the radius n,as condition for the lowest total capacity:

5 0 This equation is a result of the following mathematical thinking:The sum of the capacities between three electrodes is where 1 states thelength of the condenser. The capacity between anode and controlling gridis very small as a result of the screening effect of the screening gridand can therefore be neglected 10 without noticeable mistakes.

The stated'equation permits of a particularly suitable dimensioning notonly in the case of 35 two and three grid tubes in connection with whichthe same may preferably be made use of, but also in the ease of anotherquantity of grids.

In the drawing Fig. 1 shows the connection diagram ofa high frequencyamplifier according 40 to theinvention,

Fig. 2 a cross section of the tubes used in this amplifier,

Fig. 3 the diagram of another high frequency amplifier,

Fig. 4 a cross section of a tube used in this amplifier,

Fig. 5 the diagram of a high frequency amplifier in connection with anaudion and a loud speaker. 5

7 potential to be amplified is applied, for example,

by means of an oscillatory circuit, to the grid G1 of the first stage,so that the grid-cathode ca,- pacity of the first stage enters into theoscillatory circuit. Now the screen grid SGi is preferably disposed asfar away as possible from the plate A1, in order to make the detrimentalcapacity between these two electrodes as small as possible. A relativelylarge capacity between the screening grid SG1 and the grid Gr does notcause disturbance, because this capacity also enters into the tuning ofthe oscillatory circuit I.

In Fig. 1 there is furthermore designated by 2 the plate resistance'ofthe'first stage, by 3 the grid resistance leak of the second stage, andby 4 the connecting coupling condenser. The second stage, the output ofwhich is connected from one plate A2, for example, over a tunedtransformer 5, possesses two detrimental capacities, viz., between thecathode K2 and thegrid G2 and between the grid G2 and the screening gridSG2, while the screening grid-platepcapacity. enters into the tuning ofthe output transformer and accordingly has no detrimental effect. Inthis arrangement, with .given dimensions of the cathode and the platethe dimensioning of the screening grid will also be of a given natureinsofar as the screening grid cannot be approached to morethan a certainextent towards the plate. According to the invention, assumingcylindrical electrodes are employed, it is then preferably,

7 f with a radius Tsg of the screening grid and a radius rk'of thecathode, to so select the radius of the grid Tg that n; is equal to i Tothe tuned transformer there is connected either an additional highfrequency amplifier, a rectifier or another apparatus, which is to becontrolled by the high frequency oscillations, for example, a televisionreceiving apparatus or the like.

In Fig. 2, the two systems are illustrated diagrammatically incross-section. The designations are the same as in Fig. 1. ,1 I

In the case of limited conditions of space, and with very highfrequencies, it is frequently impossible to make the detrimentalcapacities so small that, even when'employing a screening grid, asufficient degree of amplification is capable of being obtained.desirable to compensate the'small extent of the degree of amplification,necessitated by the impossibility of obtaining a sufficiently largeplate resistance, by an appreciable slope. This is accomplished inconvenient manner by the use of a' space charge grid. An arrangement ofthis kind is illustrated in a possible form of embodiment in Figs. 3 and4. In these figures the same designations have been employed as in Figs.1 and 2. There is merelythe addition of thespace charge grids RG1 andRG2. The same considerations which were set forth in connection with theabove examples are capable of beingapplied also in this case. In thefirst system there is a single detrimental capacity between the plate A1and the screening grid SG1, so that the same should be selected as smallas possible, while the facts following: 3

In these cases it is.

capacities between the grid G1 and the screening gridSG1 between thegridG1and the space charge grid RG1, and between the space chargegridRG1 and the cathode K1 are all without detrimental effect, and with theexception of one, 5 enter into the tuning of the oscillatory circuit I.In the second system the capacities without detrimental effect areconstituted by the capacity between the screening grid SG2 and the plateA2 and between the'space charge grid RG2 and the 10 adhered to perfectlystrictly. The arrangements set forth in the above result in anamplification not obtained heretofore even in the case of certainvariation,-also in the case of frequencies at 25 which an effectiveamplification was hitherto'irnpossible by resistance amplification, 'T lV In the forms of embodiment set forth above the capa'city of twoelectrodes, which are separated by a third electrode, has not been taken30 into account'since, more particularly in the case of the narrow gridspreferably employed, this capacity as compared with the capacities ofelectrodes which are not separated by additional electrodes is so smallas to be capable of being 35 ignored. 7 r i As already stated above, theinvention is important, not only as regards aperiodic high frequencyamplifier for particularly high fre- 2,.

quencies, but also in the case of high and low 4i) frequencyemplifiersinwhich the transmission of greatly varying frequencies is to be ensuredin a manner as even as possible, such, for example, as necessary in thecase of televisioni'e- 4'5 ceiving apparatus and the like. 7 V

The amplifier system illustrated inFigures '1 to 4, including thecoupling elements employedfor connecting purposes, may-convenientlybearranged in a common vacuum space in the form at, of a multiple tube. rv

As shown by morerecent experiments, it is particularly-desirable. tomake the grid cathode .capacity of the first stage larger than in thesecond stage. The reason for this is based on the l a w y His possiblewithout difificulty in the case of the second stage, which operates withtuned plate circuit, i. e. with a relatively high plate resistance,

.to obtain a very high degree of amplification, 'which may be extendedso far that the system cannot be Lmaintainedin anon-oscillatingcondition. V I i The problem concerned accordingly consists in obtainingthe greatest possible total amplification whilst maintaining freedomfrom oscillation.

Since now the oscillation tendency of the second stage depends, greatlyon the loading of its grid and plate circuit, while on the other handthe amplification of the first tube is greatly 'depen'dent onnon-capacity of its plate circuit and the following grid circuit,.it isdesirable to increase. the amplification of the first system as far aspossible by lowering the effective grid-cathode capacity of--the.second.stage, i.. e. atthe expense "of amplification on the part ofthe second stagei'ib The amplification of the second stage isparticularly diminished by the fact that the greater grid-cathodespacing necessary for lowering the capacity decreases the slope. Thesmall grid cathode spacing of the first stage then required acts only ina favourable manner, as the slope, and accordingly the amplification, ofthis stage is particularly great. The high capacity which is caused byreason of the small spacing has no detrimental effect insofar as thesame enters into the tuning capacity of the input oscillatory circuit.

Particular advantage is furthermore obtained by selecting the diameterof the cathode to be as large as possible, in order to obtain a largeemissive surface. Since in this manner the capacity between grid andcathode is naturally considerably increased with the same grid andcathode spacing, merely the first system of the amplifier is, inaccordance with the invention, furnished with a cathode of relativelylarge diameter, while for the second system there is employed a cathodeof relatively small diameter.

The invention is described more fully with the assistance of the drawing5. In this I is an oscillatory circuit, which consists of a coil 1 and atuning condenser 8, and to which there is supplied the control energyfor example by an aerial l2 through the medium of a coil l3. Thisoscillatory circuit is connected to the grid G1 and the cathode K1 ofthe first tube of a two-stage amplifier, to the plate A1 of which is fedby the plate potential source Ea via the high resistance 2. The plate isconnected through the medium of a condenser 4 with the control grid G2of the second stage, to

which there is conducted the grid battery Eg via the resistance 3. Thecathode of this second system is designated K2. Both cathodes K1 and K2are heated indirectly by the filaments '9 and H], which may be furnishedwith heating energy from an A. 0. main through a transformer II. Theplate A2 of the second stage is connected to the source Ea via a coil 5coupled with an oscillatory circuit consisting of the coil 6 and thetuning condenser M which controls a further high frequency tube of anyparticular kind, or a detector, for example an audion. Between the gridsG1 and G2 and the appertaining plates A1 and A2 there are providedscreening grids SG1, SGz, which in the known manner are biased with a D.C. potential.

Now according to the subject matter of the 5 present invention, in thefirst place the capacity between G2 and K2 is to be smaller than thatbetween G1 and K1. According to the additional subject matter of theinvention, this is to be accomplished by the fact that the distance be-10 tween G2 and K2 is made to be greater than between G1 and K1, or thefact that the diameter of K1 is made to be larger than that of K2, otherconditions being the same.

I claim:

1. In a high-frequency amplifier two resistance-coupled tubes, each ofsaid tubes having a cathode, a plate and. a control grid, the distancebetween cathode and control grid in the second tube being larger andtherefore the capacity between said electrodes being smaller than in thefirst tube.

2. In a high frequency amplifier two resistancecoupled tubes, each ofsaid tubes having a cathode, a plate and a control grid, the diameter ofthe cathode in the second tube and therefore the capacity betweencathode and control grid being smaller than in the first tube.

3. In a high-frequency amplifier two. resistance-coupled tubes, thefirst tube having a screen-grid, the screen-grid-plate capacity beingvery small, the control-grid-plate capacity of the second tube beingsmaller than that of the first tube.

4. In a high-frequency amplifier according to claim 3 aresonance-circuit being connected to the control grid of the first tubeand another resonance circuit being connected to the plate of the secondtube.

5. In a high-frequency amplifier according to claim 3 aresonance-circuit being connected to the control grid of the first tubeand another resonance circuit being connected to the plate of the secondtube, both tubes together with the coupling-members being mounted in acommon vacuum vessel.

PAUL KAPTEYN.

